DOSSEG
.MODEL TINY
.DATA
TXT DB "Hello world!$"
.CODE
START:
	MOV ax, @DATA
	MOV ds, ax
	
	MOV ah, 09h		; prepare output function
	MOV dx, OFFSET TXT	; set offset
	INT 21h			; output string TXT
	
	MOV AX, 4C00h 		; go back to DOS
	INT 21h
END START

With A86 or NASM syntax:
  org 100h

  mov dx, msg
  mov ah, 9
  int 21h

  mov ax, 4c00h
  int 21h

msg:
  db "Hello world!$"